I bought my Gar 2 months ago feed him 2 times a day and he has gone from 7 inches to almost 9 inches, I think they slow down growing after about a year and a half but I would say about 1/2 to 1 inch a month if fed well.
A properly kept arapaima will be 4'-6' in a year.
Alligator gar aren't quite as fast growing, but you'd swear you can see either of them stretch in front of your eyes.